In recent years, there have been substantial improvements in thermodynamic cycles employing a multi-component working fluid and a combination of absorption, condensation, evaporation and recuperative heat exchange operations to reduce irreversible losses typical of conventional Rankine cycles. Generally, these improved thermodynamic cycles are known as Kalina cycles and afford demonstrable and substantial improvements in thermodynamic cycle efficiency. Kalina cycles use two interactive subsystems. The first subsystem involves a heat acquisition process for a multi-component working fluid comprising, for example, preheating, evaporating, reheating, regenerative feed heating and power generation. The second subsystem consists of a distillation/condensation subsystem (DCSS). The efficiency improvements of the Kalina cycle over the Rankine cycle are a result of the use of a multi-component working fluid, preferably an ammonia/water mixture, with the components having different boiling points at the same pressure. The compositions of the vapor and liquid streams change at different points throughout the cycle and the systems enable closer matching of the enthalpy-temperature characteristics of the working fluid and the heat source used to evaporate the working fluid and the heat sink used to condense it.
In the heat acquisition subsystem, the Kalina system closes the mismatch between the enthalpy-temperature characteristics of the heat source and working fluid as the working fluid passes through the boiler. These energy losses, typical of the Rankine cycle, are reduced by taking advantage of the changing temperature-enthalpy characteristics of the multi-component working fluid as it evaporates.
In the second subsystem, i.e., the DCSS of the Kalina cycle, the spent working fluid after expansion through the turbine, is too low in pressure and too high in ammonia concentration to be directly condensed at the temperature of available coolant. The working fluid therefore can only be partially condensed and a lean solution is mixed with a two-phase precondensed flow from a recuperative heat exchanger, thereby forming a lower concentration of ammonia/water mixture which can be fully condensed at available coolant temperature. The lean condensate is subsequently distilled recuperatively against the turbine exhaust to regenerate the working composition for the heat acquisition subsystem. The Kalina cycle has been the subject of a number of patents including U.S. Pat. Nos. 4,586,340; 4,604,867; 5,095,708 and 4,732,005, the disclosures of which are incorporated by reference.
A combined cycle power plant in its simplest form consists of a gas turbine, a steam turbine, a generator and a heat recovery steam generator (HRSG) with the gas turbine and steam turbine coupled to the single generator in tandem on a single shaft. Multi-shaft arrangements, having one or more gas turbine-generators and a common steam turbine generator have been utilized. The thermal efficiency of combined cycle plants is a function of the gas turbine performance in conjunction with the bottoming cycle. Kalina-type thermodynamic bottoming cycles have been studied for combined cycle application. Heat sources from the bottoming cycle are used for fuel heating in conventional Rankine bottoming cycle power plants. However, it was heretofore believed that the high efficiency of the Kalina bottoming cycle precluded further efficiency improvement via fuel heating.